Hard disk drives are used to store and retrieve digital information for computers and other devices. A typical hard disk drive includes a high speed rotating disk having a magnetic material on its surface. Digital information is written to and read from the disk as it rotates past a magnetic head over an air bearing interface. The magnetic head is used to detect and modify the magnetic bits on the disk's surface immediately below it. An actuator arm moves the magnetic head on an arc across the rotating disk, thereby allowing the magnetic head to access the entire disk.
In older hard disk drive designs, the bits were oriented circumferentially along the track and parallel to the disk. Today, in most hard disk drives, the bits are orientated perpendicular to the disk. These systems, known as PMR systems, reduce the size of the segment required to represent one bit of information through the perpendicular orientation of the magnetization, thereby increasing the areal density.
The magnetic head for a PMR system is designed to generate a perpendicular magnetic field. This may be achieved by embedding a soft magnetic under-layer into the disk, below the magnetic surface. In this configuration, the magnetic flux, which results from the magnetic field produced by the magnetic head, is passed through the soft magnetic under-layer and returned to the magnetic head to complete the magnetic circuit. The result is a magnetic charge with a magnetic orientation perpendicular to the surface of the disk.
The limitation of PMR is often characterized by the competing requirements of readability, writeability and stability. A problem is that to store data reliably for very small bit sizes the magnetic medium must be made of a material with a very high coercivity. At some capacity point, the bit size is so small and the coercivity correspondingly so high that the magnetic field used for writing data cannot be made strong enough to permanently affect the data and data can no longer be written to the disk.
Heat-assisted magnetic recording (HAMR) is a technology that magnetically records data on high thermal stability media using thermal assistance to first heat the material. HAMR solves this problem by temporarily and locally changing the coercivity of the magnetic storage medium by raising the temperature above the Curie temperature. Above this temperature, the medium effectively loses coercivity and a realistically achievable magnetic write field can write data to the medium. HAMR takes advantage of high-stability magnetic compounds such as iron platinum alloy. These materials can store single bits in a much smaller area without being limited by the same super paramagnetic effect that limits older technology used in hard disk storage, where the writing condition requires that the disk media must be locally heated to apply the changes in magnetic orientation at reduced coercivity.
One type of heat-assisted magnetic recording (HAMR) requires integration of a laser diode (LD) with the recording head. The laser provides light into a waveguide (WG) to energize a Near-Field Transducer (NFT) at the air bearing surface (ABS) and write pole. Metal solder bonding provides good thermal conductivities between the laser, submount and slider assemblies (to maintain stable temperature LD operation) and electric conductivities (if required) and high mechanical bond strength.
Native oxides, however, quickly form on many of the common bond materials, which can compromise the effectiveness of the bonding process and the integrity of the joint and long-term reliability. Because oxides adhere poorly to other metals, the bonding processes must break through surface oxides to establish metal-to-metal cohesion. Even after bonding, the oxides may provide a convenient site for further oxidation, leading to joint reliability and performance problems.
To assure satisfactory bonding between the submount and the slider assemblies, various procedures to block or limit the formation of native oxides may be used. Capping the solder with a gold (Au) layer has been used for preventing extreme solder oxidation. However, it is known, that tin (Sn) solder is very reactive with Au, capable of forming different AuSnx, intermetallic compounds (IMCs) (with x=4, 2, 1) at lower temperatures (even down to ambient temperature and during film deposition) and capping Sn solder with Au alone may not provide complete solder surface coverage. Exposed Sn may oxidize (which is a danger to corrosion in post-processing) and may affect the integrity of the soldering process, resulting in brittle solder bonds and voids. At the same time, excessive amounts of Au in the capping layer could results in formation of a higher Au content IMC such as Sn2Au and SnAu which consequently have a higher melting temperature (e.g., >200C) and could impact the final solder re-melting temperature.